The fibrous protein collagen accounts for 20 to 25 per cent of the total protein of mammals, where it plays important supportive roles in skins, bones, tendons and other fabrics. The immediate objective of this investigation is the determination of the manner in which collagen molecules aggregate to form the native fibrils which are the structural units of the tissues. Two principal kinds of effort are involved: (1) exploration of the consequences of the known amino-acid residue sequences of the molecular polypeptide chains in determining intermolecular associations; and (2) determination of the geometrical conditions required for fitting of the triple-chain coiled-coil molecules together. Guiding the formation of models from these considerations is the experimental information of grosser aspects of molecular and fibrillar structure as hitherto known from electron microscope and x-ray diffraction studies. In particular, the relatively direct correlation of residue sequences with the small-angle x-ray diffraction seems capable of greatly limiting the models which need to be considered from the multitude which are otherwise conceivable. Comparative studies of collagens from different sources, such as rat and kangaroo-tail tendons, teleostean fish tendon and elasmobranch shark-fin elastoidin, as well as material from lower phyla, are also involved.